1. Field of the Invention
The present invention relates generally to a lithographic apparatus and more particularly to projection systems in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.”
The radiation system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
Due to repeated, heavy use in the exposure of substrates the projection system degrades over time. Within the projection system there are individual optical elements such as refractive lenses, which can be made of quartz or flint or crown glass, or mirrors. These optical elements have coatings (the mirrors are generally embodied as substrates provided with a reflective coating), and repeated localized exposure (such as that used to expose substrates) causes specific areas of the optical element coating to change. Optically the effect is such that it appears as if the coating thins although the inventors are not entirely certain how, or why, such a change occurs. It could alternatively be due to a change in the structure of the coating. However, it will hereinafter be referred to as “thinning,” “thin” or “thinned.” The localized thinning therefore leads to aberrations in the focal plane of the optical element, and hence the projection system. In particular, the negative field curvature and astigmatism curvature of the projection system increases. As the field curvature and the astigmatism curvature of the optical elements increases the accuracy of the alignment and the exposure decreases. Prior to this invention correction and improvement of projection systems was generally only undertaken by the projection system manufacturer, thereby making it time consuming and expensive. Therefore, when the field curvature increases above a certain value the whole projection system must be replaced. Due to the high quality and accuracy necessary for the projection system, a new projection system is very expensive. It has therefore been a long felt want to provide a method of repairing the degradation due to repeated use of projection systems.
One aspect of embodiments of the present invention provides a method of at least partially repairing the degradation of projection systems due to repeated use.
This and other aspects are achieved according to embodiments of the invention by a method of decreasing focal plane anomalies of the focal plane of a projection system of a lithographic projection apparatus by flood exposing the projection system to an intense beam of radiation for a substantial period of time.
The flood exposure thins (or alternatively changes the structure of) the optical element coating. However, thinning does not occur at a constant rate and a limit is ultimately reached—further exposure does not cause further thinning of the optical element coating. Thus, the areas which were previously thinned due to substrate exposure are not thinned significantly further, but other areas are thinned to the same thickness. A uniform coating is therefore achieved and local anomalies in the focal plane due to thinning of the optical element coating are removed.
This provides a significant decrease in the field curvature. This process can therefore be applied to a projection system to improve the field curvature, and the lifetime of the projection system is significantly lengthened.
The radiation should preferably be electromagnetic radiation with a wavelength in the range 1 to 500 nm and for optimal results the projection system should be exposed to the radiation for at least 40 hours. During the exposure the reticle masking blades and aperture stops should be at their maximum potential. The intensity of the radiation should be similar to that used in substrate projection exposures. Ideally, this should be in a range of 500 to 6000 mW/cm2.
The invention should preferably be carried out in a lithographic projection apparatus comprising a radiation system for providing a projection beam of radiation. Such apparatuses have been found by the inventors to be ideally suited to this process. Moreover, as the exposure takes place in situ there is no need for time to be spent on accurate repositioning of the projection system. Furthermore, if there is more than one projection system, or another optical element for which this is a suitable process, all of the optical elements in the apparatus will be healed in a single prolonged exposure. In such a lithographic apparatus as described in the opening paragraph the substrate table is often moveable and during the extended exposure should be continuously moved in order to evenly distribute heat. A blank wafer can be placed on the substrate table to reflect light back into the projection system.
This method may find particular benefit when used in Mercury I-line apparatus.
In addition to being used to improve optical elements in which anomalies are already present a similar method can be used to prevent focal plane anomalies due to optical element coating thinning. Prior to use, the projection system is flood exposed to an intense beam of radiation for a substantial period of time. The optical element coating is therefore thinned to such an extent that further (localized projection) exposures will not thin the coating any more, and focal plane anomalies due to coating thinning are eradicated.
There can be a computer program comprising program code means that, when executed on a computer system in connection with a lithographic projection apparatus instructs the lithographic projection apparatus to measure focal plane anomalies of the focal plane in the projection system of a lithographic projection apparatus, flood expose the projection system to an intense beam of radiation for a substantial period of time, and measure again the focal plane anomalies of the projection system. The focal plane anomalies measured can be field curvature of the focal plane.
According to a further aspect of embodiments of the invention there is a dummy mask comprising a plane glass plate for diverging radiation to fill the projection system.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5–20 nm).
The term “patterning device” used herein should be broadly interpreted as referring to devices that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
In the Figures, corresponding reference symbols indicate corresponding parts.